The phaser, or phase shifter, is closely related to the
flanger in that it also works by sweeping notches through the spectrum
of the input signal. While the term phasing is sometimes used
synonymously with flanging [32],6.4 typical commercial phase shifters have been
observed to implement nonuniformly spaced notches.6.5 Furthermore, phasers such as the
Univibe (1960s) were intended to simulate the Leslie
rotating speaker effect (§5.9) as opposed to being a
low-cost analog flanging approximation. In other words, the
conceptual unification of phasing and flanging seems to be technical
in nature; i.e., based on the fact that both effects operate by
sweeping notches through the spectrum. Flangers are constrained to
have an ``infinite series'' of harmonically spaced notches
(§2.6.3), while phasers have a limited number of
nonuniformly spaced notches. In both cases motion of the notches over
time is essential to the effect, and this motion is classically
periodic. We will therefore define a phaser as any linear
filter which modulates the frequencies of a set of
non-uniformly spaced notches, while a flanger will
remain any device which modulates uniformly spaced notches.

Digital implementations of phase shifters are discussed in
§8.9. A Faust phase-shifter may be found in the
file effect.lib within the Faust software distribution
[155,171]. The Faust programming example
phaser_flanger.dsp demonstrates the phasing effect.